1. Field of the Invention
The present invention relates to bicycle frames and their method of manufacture. In particular, the invention relates to bicycle frames having frame walls which define a substantially hollow bicycle frame member made from a composite material wherein at least a portion of the frame wall is reinforced with a layer of synthetic honeycomb material sandwiched between several overlaid layers of unidirectional carbon fibers.
2. Discussion of the Related Art
Composite materials have been utilized in place of metal for frame construction in a number of applications. Such composite materials have a lower density, higher specific strength and stiffness, and better damping qualities than metals. Composite materials therefore normally provide an increase in frame strength and stiffness with a reduction in weight, as compared to metallic frames. Because of the natural desire to reduce the weight of bicycles, composite materials have also been used in the manufacture of bicycle frames.
One highly successful line of composite bicycle frames is the Trek Optimum Compaction Low Void (OCLV) series. The frame for a Trek OCLV 9000 mountain bike, for example, is formed by joining a set of composite tubes to a set of composite lugs. The tubes are those portions of the frame which form the top tube, down tube, seat tube, chain stays and seat stays. The lugs are the irregularly shaped portions of the frame which form the head lug, seat lug and bottom bracket lug. The lugs are preformed and pre-cured articles fabricated from a composite material comprised of overlaid layers of unidirectional carbon fibers embedded in an epoxy resin. Several layers of material are stacked on top of each other so that the fibers of adjacent layers cross over one another. The stacks of fibers are cut into shapes called preforms for molding into a particular lug. The preforms are placed in a mold and arranged around an inflatable bladder. The assembly of preforms is then heated and cured, with the bladder pressurized for urging the respective preforms into the shape of the mold. This method of fabrication provides a high laminate compaction having low voids which results in a strong, uniform, thin walled, tubular lug. The composite tubes may be formed by a similar method. The separate tubes and lugs are then glued together to construct the frame. Further details of this method of making an all-composite bicycle frame are disclosed in U.S. Pat. No. 5,624,519 assigned to the Trek Bicycle Corporation.
The above-described method for making composite lugs has also been used to make larger frame members, sometimes called large lugs, such as the Y-shaped frame shown in U.S. Pat. No. 5,685,553 also assigned to the Trek Bicycle Corporation. The Y-shaped frame is identified as reference numeral 12 in FIG. 1 of the '553 patent. Of course, a bicycle frame must be strong enough to support the rider's weight plus the force of bumps and jumps encountered during the ride. When a rider sits on the Y-bike or on other bicycles having similarly configured frames, the rider's weight is concentrated at the central portion or apex of the frame. When the rider applies his or her entire weight to the seat, the walls of the central portion of the Y-bike tend to bow outwardly, an effect referred to as "oil canning." The distortion of the frame walls in this manner is aesthetically undesirable and in extreme cases may lead to a failure in the frame structure. The distortion of the walls can be addressed to some degree by using a heavier grade of composite material or by adding additional layers of material to thicken the walls, but the added material obviously adds weight to the bicycle.
Of course, in the field of bicycles, and especially in the field of bicycle racing there is a never-ending desire to build ever lighter bicycles and bicycle frames.